It depends on how traffic tolerant you are, but I found the roads to be fine. Some of it is fairly remote, at one point there were even free range cattle on the roadway. The terrain is quite variable and interesting.
May can be pretty wet on the coast though.
I didn't care for the Washington portion of my Seattle to San Luis Obispo ride, but it was a relatively small part of the ride.
It probably isn't even an option at that time anyway, but I will say that the SC route is extremely challenging. It is also very hard to pick a time and direction of travel that will not expose you to some pretty extreme temperatures. We had 110+ F and overnight frost in the same 48 hour period. We started in June in the south and still found services not yet open in most of Yosemite. My guess would be that starting in the north would require an even later start, but I have only ridden the southern half S-N.